Render result does not appear when file is reopened
Blender does not save the render result with the file.  If you close the file without saving the render it will be lost.



You can save a rendered image by going to Image > Save Image in the UV/Image Editor (hotkey Alt+S).



If you need to save all the render layers and passes with extended dynamic range for future post-processing in Blender, you can save to a Multilayer Open-EXR image file and import it into the compositor next time with an Image node.





Also see: How to save the render result within Blender.